Read refresh to improve power on data retention for a non-volatile memory

ABSTRACT

A memory apparatus and method of operation are provided. The apparatus includes memory cells each connected to a word line and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to a data state. The memory cells are operable in one of a first read condition in which a word line voltage is discharged and a second read condition in which the word line voltage is coupled up to a residual voltage level. A control circuit determines a power on event and periodically apply a predetermined refresh read voltage to the word line for a predetermined period of time for each of the plurality of blocks at a specified interval based on at least one data retention factor to maintain the memory cells of the plurality of blocks in the second read condition in response to determining the power on event.

FIELD

This application relates to non-volatile memory apparatuses and the operation of non-volatile memory apparatuses.

BACKGROUND

This section provides background information related to the technology associated with the present disclosure and, as such, is not necessarily prior art.

Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices.

A charge-storing material such as a floating gate or a charge-trapping material can be used in such memory devices to store a charge which represents a data state. A charge-trapping material can be arranged vertically in a three-dimensional (3D) stacked memory structure, or horizontally in a two-dimensional (2D) memory structure. One example of a 3D memory structure is the Bit Cost Scalable (BiCS) architecture which comprises a stack of alternating conductive and dielectric layers.

A memory device includes memory cells which may be arranged in strings, for instance, where select gate transistors are provided at the ends of the string to selectively connect a channel of the string to a source line or bit line. However, various challenges are presented in operating such memory devices.

SUMMARY

This section provides a general summary of the present disclosure and is not a comprehensive disclosure of its full scope or all of its features and advantages.

An object of the present disclosure is to provide a memory apparatus and a method of operating the memory apparatus that address and overcome the above-noted shortcomings.

Accordingly, it is an aspect of the present disclosure to provide an apparatus including a plurality of memory cells. Each of the plurality of memory cells is connected to one of a plurality of word lines and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to one of a plurality of data states. The plurality of memory cells are operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up to a residual voltage level. A control circuit is coupled to the plurality of word lines and configured to determine a power on event of the apparatus in which the plurality of memory cells are in the first read condition. The control circuit is additionally configured to periodically apply a predetermined refresh read voltage to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the apparatus.

According to another aspect of the disclosure, a controller in communication with a memory apparatus including a plurality of memory cells. Each of the plurality of memory cells is connected to one of a plurality of word lines and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to one of a plurality of data states. The plurality of memory cells are operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up to a residual voltage level. The controller is configured to determine a power on event of the memory apparatus in which the plurality of memory cells are in the first read condition. The controller is also configured to instruct the memory apparatus to periodically apply a predetermined refresh read voltage to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the memory apparatus.

According to an additional aspect of the disclosure a method of operating a memory apparatus is provided. The memory apparatus includes a plurality of memory cells. Each of the plurality of memory cells is connected to one of a plurality of word lines and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to one of a plurality of data states. The plurality of memory cells are operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up to a residual voltage level. The method includes the step of determining a power on event of the memory apparatus in which the plurality of memory cells are in the first read condition. The method continues with the step of periodically applying a predetermined refresh read voltage to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the memory apparatus.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1A is a block diagram of an example memory apparatus or device according to aspects of the disclosure;

FIG. 1B depicts an example memory cell according to aspects of the disclosure;

FIG. 1C depicts an example of the temperature-sensing circuit of FIG. 1A according to aspects of the disclosure;

FIG. 2 is a block diagram of the example memory device, depicting additional details of the controller according to aspects of the disclosure;

FIG. 3 is a perspective view of a memory device comprising a set of blocks in an example 3D configuration of the memory structure of FIG. 1 according to aspects of the disclosure;

FIG. 4 depicts an example cross-sectional view of a portion of one of the blocks of FIG. 3 according to aspects of the disclosure;

FIG. 5 depicts a plot of memory hole/pillar diameter in the stack of FIG. 4 according to aspects of the disclosure;

FIG. 6 depicts a close-up view of a region of the stack of FIG. 4 according to aspects of the disclosure;

FIG. 7A depicts an example view of NAND strings in sub-blocks in a 3D configuration which is consistent with FIG. 4 according to aspects of the disclosure;

FIG. 7B depicts word line and SGD layers in an example set of blocks which is consistent with FIG. 4 according to aspects of the disclosure;

FIG. 8A depicts an example threshold voltage (Vth) distribution of memory cells, where eight data states are used, in a first read condition compared to a second read condition according to aspects of the disclosure;

FIG. 8B depicts example bit sequences for lower, middle and upper pages of data, and associated read voltages, for the Vth distributions of FIG. 8A according to aspects of the disclosure;

FIG. 9 depicts a waveform of an example programming operation according to aspects of the disclosure;

FIG. 10A depicts a plot of example waveforms in a programming operation, showing coupling up of a word line voltage according to aspects of the disclosure;

FIG. 10B depicts a plot of a channel voltage (Vch) corresponding to FIG. 10 according to aspects of the disclosure;

FIG. 10C depicts a plot of example waveforms in a read operation, showing coupling up of a word line voltage according to aspects of the disclosure;

FIG. 10D depicts a plot of a channel voltage (Vch) corresponding to FIG. 10C according to aspects of the disclosure;

FIG. 10E depicts the waveforms of FIG. 10C showing a decay of the coupled up voltage of the word line according to aspects of the disclosure;

FIG. 10F depicts a plot of a channel voltage consistent with FIG. 10E according to aspects of the disclosure;

FIG. 10G depicts a plot of a Vth of a memory cell connected to the coupled up word line, consistent with FIGS. 10E and 10F according to aspects of the disclosure;

FIG. 11A depicts control gate and channel voltages on a memory cell which acts as a capacitor when the control gate voltage is decreased in a sensing operation according to aspects of the disclosure;

FIG. 11B depicts a portion of a memory cell, showing electron injection into a charge trapping region during weak programming according to aspects of the disclosure;

FIG. 12 shows a refresh frequency and temperature lookup table including a plurality of specified intervals each corresponding to one of a plurality of magnitudes of die temperature according to aspects of the disclosure;

FIGS. 13A and 13B show the threshold voltage possible for each of a plurality of memory cells spanning a threshold window with each of the plurality of memory cells can be configured to store a plurality of bits according to aspects of the disclosure;

FIG. 14 shows distributions of the threshold voltage of the fourteenth data state and a fifteenth data state before and after a check of data retention according to aspects of the disclosure;

FIG. 15 illustrates steps of a method of operating the memory apparatus according to aspects of the disclosure;

FIGS. 16A-16B show a sector fail rate (SFR) versus failure bit count (FBC)/sector for varying data retention periods of time with and without a refresh or dummy read for an example memory apparatus according to aspects of the disclosure;

FIG. 17 shows threshold voltage distributions for an example memory apparatus after 80 hour data retention 85 degrees Celsius bake with 2000 write/erase cycles with and without the refresh or dummy read according to aspects of the disclosure;

FIG. 18 shows a threshold voltage margin improvement for data retention with the refresh read for an example memory apparatus according to aspects of the disclosure; and

FIG. 19 shows threshold voltage distributions for an example memory apparatus after 80 hour data retention 85 degrees Celsius bake with various write/erase cycles with and without the refresh or dummy read according to aspects of the disclosure.

To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.

DETAILED DESCRIPTION

In the following description, details are set forth to provide an understanding of the present disclosure. In some instances, certain circuits, structures and techniques have not been described or shown in detail in order not to obscure the disclosure.

In general, the present disclosure relates to non-volatile memory apparatuses of the type well-suited for use in many applications. The non-volatile memory apparatus and associated methods of operation of this disclosure will be described in conjunction with one or more example embodiments. However, the specific example embodiments disclosed are merely provided to describe the inventive concepts, features, advantages and objectives with sufficient clarity to permit those skilled in this art to understand and practice the disclosure. Specifically, the example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

In some memory devices, memory cells are joined to one another such as in NAND strings in a block or sub-block. Each NAND string comprises a number of memory cells connected in series between one or more drain-side SG transistors (SGD transistors), on a drain-side of the NAND string which is connected to a bit line, and one or more source-side SG transistors (SGS transistors), on a source-side of the NAND string which is connected to a source line. Further, the memory cells can be arranged with a common control gate line (e.g., word line) which acts a control gate. A set of word lines extends from the source side of a block to the drain side of a block. Memory cells can be connected in other types of strings and in other ways as well.

The memory cells can include data memory cells, which are eligible to store user data, and dummy or non-data memory cells which are ineligible to store user data. A dummy word line is connected to a dummy memory cell. One or more dummy memory cells may be provided at the drain and/or source ends of a string of memory cells to provide a gradual transition in channel gradient.

During a programming operation, the memory cells are programmed according to a word line programming order. For example, the programming may start at the word line at the source side of the block and proceed to the word line at the drain side of the block. In one approach, each word line is completely programmed before programming a next word line. For example, a first word line, WL0, is programmed using one or more programming passes until the programming is completed. Next, a second word line, WL1, is programmed using one or more programming passes until the programming is completed, and so forth. A programming pass may include a set of increasing program voltages which are applied to the word line in respective program loops or program-verify iterations, such as depicted in FIG. 9. Verify operations may be performed after each program voltage to determine whether the memory cells have completed programming. When programming is completed for a memory cell, it can be locked out from further programming while programming continues for other memory cells in subsequent program loops.

The memory cells may also be programmed according to a sub-block programming order, where memory cells in one sub-block, or portion of a block, are programmed before programming memory cells in another sub-block.

Each memory cell may be associated with a data state according to write data in a program command. Based on its data state, a memory cell will either remain in the erased state or be programmed to a programmed data state. For example, in a one bit per cell memory device, there are two data states including the erased state and the programmed state. In a two-bit per cell memory device, there are four data states including the erased state and three higher data states referred to as the A, B and C data states. In a three-bit per cell memory device, there are eight data states including the erased state and seven higher data states referred to as the A, B, C, D, E, F and G data states (see FIG. 8A). In a four-bit per cell memory device, there are sixteen data states including the erased state and fifteen higher data states. The data states may be referred to as the S0, S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S13, S14 and S15 data states where S0 is the erased state.

After the memory cells are programmed, the data can be read back in a read operation. A read operation can involve applying a series of read voltages to a word line while sensing circuitry determines whether cells connected to the word line are in a conductive or non-conductive state. If a cell is in a non-conductive state, the Vth of the memory cell exceeds the read voltage. The read voltages are set at levels which are expected to be between the threshold voltage levels of adjacent data states.

However, it has been observed that the Vth of a memory cell can vary depending on when the read operation occurs. For example, the Vth can vary in the memory cells depending on a coupled up state of the word lines when the read operation occurs. A “first read” condition can be defined in which the word lines are not coupled up, and a “second read” condition can be defined in which the word lines are coupled up.

The memory cells can be in the first read condition after a power on event in the memory device. When the memory device is powered up for use, an operation may occur which checks for bad blocks. This operation involves applying 0 V or other low voltage to the word lines. As a result, any coupling up of the word line voltages is discharged.

The word lines can also be discharged in a block when the word line voltages are set to a low level. This can occur when the block is inactive while an operation is performed in another block. The cells can also be in the first read condition after a significant amount of time has passed after a last sensing operation, since the word lines discharge over time. The coupling up of the word lines causes a Vth shift in the cells due to inadvertent programming or erasing. Since the word lines are not significantly coupled up while in the first read condition, this Vth does not occur.

The cells can be in the second read condition when the read occurs shortly, e.g., seconds or minutes, after a last sensing operation. Since the word lines are relatively strongly coupled up while in the second read condition, there is a programming or erasing of the cells due to the word line voltage, and a corresponding shift in the Vth. In particular, the word lines with a coupled-up voltage can cause weak programming of cells which have a relatively low Vth, lower than the coupled-up voltage, e.g., cells in lower programmed data states, thus resulting in a Vth upshift for these cells. Also, there can be a weak erasing of cells which have a relatively high Vth, higher than the coupled-up voltage, e.g., cells in higher programmed data states, thus resulting in a Vth downshift for these cells.

The cells gradually transition from the second read condition to the first read condition over time, e.g., one hour, as the word lines are discharged.

The coupling up of the word line voltage is caused by the voltages of a sensing operation such as a verify operation which occurs in connection with a programming operation, or a read operation which occurs after a programming operation is completed. The sensing of the cells involves the application of a sensing voltage (e.g., a read/verify voltage) to a selected word line. At the same time, a read pass voltage is applied to the unselected word lines and then stepped down. This step down temporarily reduces a channel voltage due to capacitive coupling. When the channel voltage increases back to its nominal level, this causes an increase or coupling up of the word line voltages, also due to capacitive coupling. For cells in the lower data states, the Vth gradually decreases as electrons which are trapped in the charge trapping material of the cells are de-trapped and return to the channel. For cells in the higher data states, the Vth gradually increases as electrons are removed from the channel. See FIG. 8A. Techniques provided herein address the above and other issues.

FIG. 1A is a block diagram of an example memory device. The memory device 100, such as a non-volatile storage system, may include one or more memory die 108. The memory die 108 includes a memory structure 126 of memory cells, such as an array of memory cells, control circuitry 110, and read/write circuits 128. The memory structure 126 is addressable by word lines via a row decoder 124 and by bit lines via a column decoder 132. The read/write circuits 128 include multiple sense blocks 51, 52, . . . , 53 (sensing circuitry) and allow a page of memory cells to be read or programmed in parallel. Typically a controller 122 is included in the same memory device 100 (e.g., a removable storage card) as the one or more memory die 108. The controller may be separate from the memory die. Commands and data are transferred between the host 140 and controller 122 via a data bus 120, and between the controller and the one or more memory die 108 via lines 118.

The memory structure can be 2D or 3D. The memory structure may comprise one or more array of memory cells including a 3D array. The memory structure may comprise a monolithic 3D memory structure in which multiple memory levels are formed above (and not in) a single substrate, such as a wafer, with no intervening substrates. The memory structure may comprise any type of non-volatile memory that is monolithically formed in one or more physical levels of arrays of memory cells having an active area disposed above a silicon substrate. The memory structure may be in a non-volatile memory device having circuitry associated with the operation of the memory cells, whether the associated circuitry is above or within the substrate.

The control circuitry 110 cooperates with the read/write circuits 128 to perform memory operations on the memory structure 126, and includes a state machine 112, an on-chip address decoder 114, and a power control module 116. The state machine 112 provides chip-level control of memory operations. The state machine may include a clock 112 a to determine an elapsed time since a last sensing operation, as discussed further below. A storage region 113 may be provided, e.g., for sets of read voltage, as described further below. Generally, the storage region may store operational parameters and software/code. A timer 113 a may also be used to determine timing of predetermined refresh read voltage pulses to word lines, described in more detail below, for example. A temperature sensor 115 may also be provided.

In one embodiment, the state machine is programmable by the software. In other embodiments, the state machine does not use software and is completely implemented in hardware (e.g., electrical circuits).

The on-chip address decoder 114 provides an address interface between that used by the host or a memory controller to the hardware address used by the decoders 124 and 132. The power control module 116 controls the power and voltages supplied to the word lines, select gate lines and bit lines during memory operations. It can include drivers for word lines, SGS and SGD transistors and source lines. The sense blocks can include bit line drivers, in one approach. An SGS transistor is a select gate transistor at a source end of a NAND string, and an SGD transistor is a select gate transistor at a drain end of a NAND string.

In some implementations, some of the components can be combined. In various designs, one or more of the components (alone or in combination), other than memory structure 126, can be thought of as at least one control circuit which is configured to perform the techniques described herein including the steps of the processes described herein. For example, a control circuit may include any one of, or a combination of, control circuitry 110, state machine 112, decoders 114 and 132, power control module 116, sense blocks 51, 52, . . . , 53, read/write circuits 128, controller 122, and so forth.

The off-chip controller 122 (which in one embodiment is an electrical circuit) may comprise a processor 122 c, storage devices (memory) such as ROM 122 a and RAM 122 b and an error-correction code (ECC) engine 245. The ECC engine can correct a number of read errors.

A memory interface 122 d may also be provided. The memory interface, in communication with ROM, RAM and processor, is an electrical circuit that provides an electrical interface between controller and memory die. For example, the memory interface can change the format or timing of signals, provide a buffer, isolate from surges, latch I/O and so forth. The processor can issue commands to the control circuitry 110 (or any other component of the memory die) via the memory interface 122 d.

The storage device comprises code such as a set of instructions, and the processor is operable to execute the set of instructions to provide the functionality described herein. Alternatively or additionally, the processor can access code from a storage device 126 a of the memory structure, such as a reserved area of memory cells in one or more word lines.

For example, code can be used by the controller to access the memory structure such as for programming, read and erase operations. The code can include boot code and control code (e.g., a set of instructions). The boot code is software that initializes the controller during a booting or startup process and enables the controller to access the memory structure. The code can be used by the controller to control one or more memory structures. Upon being powered up, the processor 122 c fetches the boot code from the ROM 122 a or storage device 126 a for execution, and the boot code initializes the system components and loads the control code into the RAM 122 b. Once the control code is loaded into the RAM, it is executed by the processor. The control code includes drivers to perform basic tasks such as controlling and allocating memory, prioritizing the processing of instructions, and controlling input and output ports.

Generally, the control code can include instructions to perform the functions described herein including the steps of the flowcharts discussed further below, and provide the voltage waveforms including those discussed further below. A control circuit can be configured to execute the instructions to perform the functions described herein.

In one embodiment, the host is a computing device (e.g., laptop, desktop, smartphone, tablet, digital camera) that includes one or more processors, one or more processor readable storage devices (RAM, ROM, flash memory, hard disk drive, solid state memory) that store processor readable code (e.g., software) for programming the one or more processors to perform the methods described herein. The host may also include additional system memory, one or more input/output interfaces and/or one or more input/output devices in communication with the one or more processors.

Other types of non-volatile memory in addition to NAND flash memory can also be used.

Semiconductor memory devices include volatile memory devices, such as dynamic random access memory (“DRAM”) or static random access memory (“SRAM”) devices, non-volatile memory devices, such as resistive random access memory (“ReRAM”), electrically erasable programmable read only memory (“EEPROM”), flash memory (which can also be considered a subset of EEPROM), ferroelectric random access memory (“FRAM”), and magnetoresistive random access memory (“MRAM”), and other semiconductor elements capable of storing information. Each type of memory device may have different configurations. For example, flash memory devices may be configured in a NAND or a NOR configuration.

The memory devices can be formed from passive and/or active elements, in any combinations. By way of non-limiting example, passive semiconductor memory elements include ReRAM device elements, which in some embodiments include a resistivity switching storage element, such as an anti-fuse or phase change material, and optionally a steering element, such as a diode or transistor. Further by way of non-limiting example, active semiconductor memory elements include EEPROM and flash memory device elements, which in some embodiments include elements containing a charge storage region, such as a floating gate, conductive nanoparticles, or a charge storage dielectric material.

Multiple memory elements may be configured so that they are connected in series or so that each element is individually accessible. By way of non-limiting example, flash memory devices in a NAND configuration (NAND memory) typically contain memory elements connected in series. A NAND string is an example of a set of series-connected transistors comprising memory cells and SG transistors.

A NAND memory array may be configured so that the array is composed of multiple strings of memory in which a string is composed of multiple memory elements sharing a single bit line and accessed as a group. Alternatively, memory elements may be configured so that each element is individually accessible, e.g., a NOR memory array. NAND and NOR memory configurations are examples, and memory elements may be otherwise configured.

The semiconductor memory elements located within and/or over a substrate may be arranged in two or three dimensions, such as a 2D memory structure or a 3D memory structure.

In a 2D memory structure, the semiconductor memory elements are arranged in a single plane or a single memory device level. Typically, in a 2D memory structure, memory elements are arranged in a plane (e.g., in an x-y direction plane) which extends substantially parallel to a major surface of a substrate that supports the memory elements. The substrate may be a wafer over or in which the layer of the memory elements are formed or it may be a carrier substrate which is attached to the memory elements after they are formed. As a non-limiting example, the substrate may include a semiconductor such as silicon.

The memory elements may be arranged in the single memory device level in an ordered array, such as in a plurality of rows and/or columns. However, the memory elements may be arrayed in non-regular or non-orthogonal configurations. The memory elements may each have two or more electrodes or contact lines, such as bit lines and word lines.

A 3D memory array is arranged so that memory elements occupy multiple planes or multiple memory device levels, thereby forming a structure in three dimensions (i.e., in the x, y and z directions, where the z direction is substantially perpendicular and the x and y directions are substantially parallel to the major surface of the substrate).

As a non-limiting example, a 3D memory structure may be vertically arranged as a stack of multiple 2D memory device levels. As another non-limiting example, a 3D memory array may be arranged as multiple vertical columns (e.g., columns extending substantially perpendicular to the major surface of the substrate, i.e., in the y direction) with each column having multiple memory elements. The columns may be arranged in a 2D configuration, e.g., in an x-y plane, resulting in a 3D arrangement of memory elements with elements on multiple vertically stacked memory planes. Other configurations of memory elements in three dimensions can also constitute a 3D memory array.

By way of non-limiting example, in a 3D NAND memory array, the memory elements may be coupled together to form a NAND string within a single horizontal (e.g., x-y) memory device level. Alternatively, the memory elements may be coupled together to form a vertical NAND string that traverses across multiple horizontal memory device levels. Other 3D configurations can be envisioned wherein some NAND strings contain memory elements in a single memory level while other strings contain memory elements which span through multiple memory levels. 3D memory arrays may also be designed in a NOR configuration and in a ReRAM configuration.

Typically, in a monolithic 3D memory array, one or more memory device levels are formed above a single substrate. Optionally, the monolithic 3D memory array may also have one or more memory layers at least partially within the single substrate. As a non-limiting example, the substrate may include a semiconductor such as silicon. In a monolithic 3D array, the layers constituting each memory device level of the array are typically formed on the layers of the underlying memory device levels of the array. However, layers of adjacent memory device levels of a monolithic 3D memory array may be shared or have intervening layers between memory device levels.

2D arrays may be formed separately and then packaged together to form a non-monolithic memory device having multiple layers of memory. For example, non-monolithic stacked memories can be constructed by forming memory levels on separate substrates and then stacking the memory levels atop each other. The substrates may be thinned or removed from the memory device levels before stacking, but as the memory device levels are initially formed over separate substrates, the resulting memory arrays are not monolithic 3D memory arrays. Further, multiple 2D memory arrays or 3D memory arrays (monolithic or non-monolithic) may be formed on separate chips and then packaged together to form a stacked-chip memory device.

Associated circuitry is typically required for operation of the memory elements and for communication with the memory elements. As non-limiting examples, memory devices may have circuitry used for controlling and driving memory elements to accomplish functions such as programming and reading. This associated circuitry may be on the same substrate as the memory elements and/or on a separate substrate. For example, a controller for memory read-write operations may be located on a separate controller chip and/or on the same substrate as the memory elements.

One of skill in the art will recognize that this technology is not limited to the 2D and 3D exemplary structures described but covers all relevant memory structures within the spirit and scope of the technology as described herein and as understood by one of skill in the art.

FIG. 1B depicts an example memory cell 200. The memory cell comprises a control gate CG which receives a word line voltage Vwl, a drain at a voltage Vd, a source at a voltage Vs and a channel at a voltage Vch.

FIG. 1C depicts an example of the temperature-sensing circuit 115 of FIG. 1A. The circuit includes pMOSFETs 131 a, 131 b and 134, bipolar transistors 133 a and 133 b and resistors R1, R2 and R3. I1, I2 and I3 denote currents. Voutput is a temperature-based output voltage provided to an analog-to-digital (ADC) converter 129. Vbg is a temperature-independent voltage. A voltage level generation circuit 135 uses Vbg to set a number of voltage levels. For example, a reference voltage may be divided down into several levels by a resistor divider circuit.

The ADC compares Voutput to the voltage levels and selects a closest match among the voltage levels, outputting a corresponding digital value (VTemp) to the processor. This is data indicating a temperature of the memory device. ROM fuses 123 store data which correlates the matching voltage level to a temperature, in one approach. The processor then uses the temperature to set temperature-based parameters in the memory device.

Vbg, is obtained by adding the base-emitter voltage (Vbe) across the transistor 131 b and the voltage drop across the resistor R2. The bipolar transistor 133 a has a larger area (by a factor N) than the transistor 133 b. The PMOS transistors 131 a and 131 b are equal in size and are arranged in a current mirror configuration so that the currents I1 and I2 are substantially equal. We have Vbg=Vbe+R2×I2 and I1=Ve/R1 so that I2=Ve/R1. As a result, Vbg=Vbe+R2×kT In(N)/R1×q, where T is temperature, k is Boltzmann's constant and q is a unit of electric charge. The source of the transistor 134 is connected to a supply voltage Vdd and the node between the transistor's drain and the resistor R3 is the output voltage, Voutput. The gate of the transistor 134 is connected to the same terminal as the gates of transistors 131 a and 131 b and the current through the transistor 134 mirrors the current through the transistors 131 a and 131 b.

FIG. 2 is a block diagram of the example memory device 100, depicting additional details of the controller 122. As used herein, a flash memory controller is a device that manages data stored on flash memory and communicates with a host, such as a computer or electronic device. A flash memory controller can have various functionality in addition to the specific functionality described herein. For example, the flash memory controller can format the flash memory to ensure the memory is operating properly, map out bad flash memory cells, and allocate spare memory cells to be substituted for future failed cells. Some part of the spare cells can be used to hold firmware to operate the flash memory controller and implement other features. In operation, when a host needs to read data from or write data to the flash memory, it will communicate with the flash memory controller. If the host provides a logical address to which data is to be read/written, the flash memory controller can convert the logical address received from the host to a physical address in the flash memory. (Alternatively, the host can provide the physical address). The flash memory controller can also perform various memory management functions, such as, but not limited to, wear leveling (distributing writes to avoid wearing out specific blocks of memory that would otherwise be repeatedly written to) and garbage collection (after a block is full, moving only the valid pages of data to a new block, so the full block can be erased and reused).

The interface between the controller 122 and non-volatile memory die 108 may be any suitable flash interface. In one embodiment, memory device 100 may be a card based system, such as a secure digital (SD) or a micro secure digital (micro-SD) card. In an alternate embodiment, the memory system may be part of an embedded memory system. For example, the flash memory may be embedded within the host, such as in the form of a solid state disk (SSD) drive installed in a personal computer.

In some embodiments, the memory device 100 includes a single channel between the controller 122 and the non-volatile memory die 108, the subject matter described herein is not limited to having a single memory channel.

The controller 122 includes a front end module 208 that interfaces with a host, a back end module 210 that interfaces with the one or more non-volatile memory die 108, and various other modules that perform functions which will now be described in detail.

The components of the controller may take the form of a packaged functional hardware unit (e.g., an electrical circuit) designed for use with other components, a portion of a program code (e.g., software or firmware) executable by a processor, e.g., microprocessor, or processing circuitry that usually performs a particular function of related functions, or a self-contained hardware or software component that interfaces with a larger system, for example. For example, each module may include an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a circuit, a digital logic circuit, an analog circuit, a combination of discrete circuits, gates, or any other type of hardware or combination thereof. Alternatively or in addition, each module may include software stored in a processor readable device (e.g., memory) to program a processor for the controller to perform the functions described herein. The architecture depicted in FIG. 2 is one example implementation that may (or may not) use the components of the controller 122 depicted in FIG. 1A (e.g., RAM, ROM, processor, interface).

The controller 122 may include recondition circuitry 212, which is used for reconditioning memory cells or blocks of memory. The reconditioning may include refreshing data in its current location or reprogramming data into a new word line or block as part of performing erratic word line maintenance, as described below.

Referring again to modules of the controller 122, a buffer manager/bus controller 214 manages buffers in random access memory (RAM) 216 and controls the internal bus arbitration of Controller 122. The RAM may include DRAM and/or SRAM. DRAM or Dynamic Random Access Memory is a type of semiconductor memory in which the memory is stored in the form of a charge. Each memory cell in a DRAM is made of a transistor and a capacitor. The data is stored in the capacitor. Capacitors loose charge due to leakage and hence DRAMs are volatile devices. To keep the data in the memory, the device must be regularly refreshed. In contrast, SRAM or Static Random Access Memory will retain a value as long as power is supplied.

A read only memory (ROM) 218 stores system boot code. Although illustrated in FIG. 2 as being located separately from the controller, in other embodiments, one or both of the RAM 216 and ROM 218 may be located within the controller. In yet other embodiments, portions of RAM and ROM may be located both within the controller 122 and outside the controller. Further, in some implementations, the controller 122, RAM 216, and ROM 218 may be located on separate semiconductor die.

Front end module 208 includes a host interface 220 and a physical layer interface (PHY) 222 that provide the electrical interface with the host or next level storage controller. The choice of the type of host interface 220 can depend on the type of memory being used. Examples of host interfaces 220 include, but are not limited to, SATA, SATA Express, SAS, Fibre Channel, USB, PCIe, and NVMe. The host interface 220 typically facilitates transfer for data, control signals, and timing signals.

Back end module 210 includes an error correction controller (ECC) engine 224 that encodes the data bytes received from the host, and decodes and error corrects the data bytes read from the non-volatile memory. A command sequencer 226 generates command sequences, such as program and erase command sequences, to be transmitted to non-volatile memory die 108. A RAID (Redundant Array of Independent Dies) module 228 manages generation of RAID parity and recovery of failed data. The RAID parity may be used as an additional level of integrity protection for the data being written into the memory device 100. In some cases, the RAID module 228 may be a part of the ECC engine 224. Note that the RAID parity may be added as an extra die or dies as implied by the common name, but it may also be added within the existing die, e.g. as an extra plane, or extra block, or extra word lines within a block. A memory interface 230 provides the command sequences to non-volatile memory die 108 and receives status information from the non-volatile memory die. A flash control layer 232 controls the overall operation of back end module 210.

Additional components of memory device 100 include media management layer 238, which performs wear leveling of memory cells of non-volatile memory die 108. The memory system also includes other discrete components 240, such as external electrical interfaces, external RAM, resistors, capacitors, or other components that may interface with controller 122. In alternative embodiments, one or more of the physical layer interface 222, RAID module 228, media management layer 238 and buffer management/bus controller 214 are optional components that are not necessary in the controller 122.

The Flash Translation Layer (FTL) or Media Management Layer (MML) 238 may be integrated as part of the flash management that may handle flash errors and interfacing with the host. In particular, MML may be a module in flash management and may be responsible for the internals of NAND management. In particular, the MML 238 may include an algorithm in the memory device firmware which translates writes from the host into writes to the memory structure 126, e.g., flash memory, of die 108. The MML 238 may be needed because: 1) the flash memory may have limited endurance; 2) the flash memory may only be written in multiples of pages; and/or 3) the flash memory may not be written unless it is erased as a block. The MML 238 understands these potential limitations of the flash memory which may not be visible to the host. Accordingly, the MML 238 attempts to translate the writes from host into writes into the flash memory. Erratic bits may be identified and recorded using the MML 238. This recording of erratic bits can be used for evaluating the health of blocks and/or word lines (the memory cells on the word lines).

The controller 122 may interface with one or more memory dies 108. In one embodiment, the controller and multiple memory dies (together comprising the memory device 100) implement a solid state drive (SSD), which can emulate, replace or be used instead of a hard disk drive inside a host, as a network-attached storage (NAS) device, and so forth. Additionally, the SSD need not be made to work as a hard drive.

FIG. 3 is a perspective view of a memory device 600 comprising a set of blocks in an example 3D configuration of the memory structure 126 of FIG. 1A. On the substrate are example blocks BLK0, BLK1, BLK2 and BLK3 of memory cells (storage elements) and a peripheral area 604 with circuitry for use by the blocks. For example, the circuitry can include voltage drivers 605 which can be connected to control gate layers of the blocks. In one approach, control gate layers at a common height in the blocks are commonly driven. The substrate 601 can also carry circuitry under the blocks, along with one or more lower metal layers which are patterned in conductive paths to carry signals of the circuitry. The blocks are formed in an intermediate region 602 of the memory device. In an upper region 603 of the memory device, one or more upper metal layers are patterned in conductive paths to carry signals of the circuitry. Each block comprises a stacked area of memory cells, where alternating levels of the stack represent word lines. In one possible approach, each block has opposing tiered sides from which vertical contacts extend upward to an upper metal layer to form connections to conductive paths. While four blocks are depicted as an example, two or more blocks can be used, extending in the x- and/or y-directions.

In one possible approach, the blocks are in a plane, and the length of the plane, in the x-direction, represents a direction in which signal paths to word lines extend in the one or more upper metal layers (a word line or SGD line direction), and the width of the plane, in the y-direction, represents a direction in which signal paths to bit lines extend in the one or more upper metal layers (a bit line direction). The z-direction represents a height of the memory device. The blocks could also be arranged in multiple planes.

FIG. 4 depicts an example cross-sectional view of a portion of one of the blocks of FIG. 3. The block comprises a stack 616 of alternating conductive and dielectric layers. In this example, the conductive layers comprise two SGD layers, two SGS layers and four dummy word line layers (or word lines) WLD1, WLD2, WLD3 and WLD4, in addition to data word line layers (or word lines) WLL0-WLL10. The dielectric layers are labelled as DL0-DL19. Further, regions of the stack which comprise NAND strings NS1 and NS2 are depicted. Each NAND string encompasses a memory hole 618 or 619 which is filled with materials which form memory cells adjacent to the word lines. A region 622 of the stack is shown in greater detail in FIG. 6.

The stack includes a substrate 611. In one approach, a portion of the source line SL comprises an n-type source diffusion layer 611 a in the substrate which is in contact with a source end of each string of memory cells in a block. The n-type source diffusion layer 611 a is formed in a p-type well region 611 b, which in turn is formed in an n-type well region 611 c, which in turn is formed in a p-type semiconductor substrate 611 d, in one possible implementation. The n-type source diffusion layer may be shared by all of the blocks in a plane, in one approach.

NS1 has a source-end 613 at a bottom 616 b of the stack and a drain-end 615 at a top 616 a of the stack. Local interconnects, such as local interconnect 617, may be provided periodically across the stack. The local interconnects may be metal-filled slits which extend through the stack, such as to connect the source line/substrate to a line above the stack. The slits may be used during the formation of the word lines and subsequently filled with metal. The local interconnect comprises a conductive region 617 a (e.g., metal) within an insulating region 617 b. A portion of a bit line BL0 is also depicted. A conductive via 621 connects the drain-end 615 of NS1 to BL0.

In one approach, the block of memory cells comprises a stack of alternating control gate and dielectric layers, and the memory cells are arranged in vertically extending memory holes in the stack.

In one approach, each block comprises a terraced edge in which vertical interconnects connect to each layer, including the SGS, WL and SGD layers, and extend upward to horizontal paths to voltage sources.

This example includes two SGD transistors, two drain side dummy memory cells, two source side dummy memory cells and two SGS transistors in each string, as an example. Generally, the use of dummy memory cells is optional and one or more may be provided. Also, one or more SGD transistors and one or more SGS transistors may be provided in a memory string.

An insulating region 620 may be provided to separate portions of the SGD layers from one another to provide one independently driven SGD line per sub-block. In this example, the word line layers are common to two adjacent sub-blocks. See also FIG. 7B. In another possible implementation, the insulating region 620 extends down to the substrate to separate the word line layers. In this case, the word line layers are separate in each sub-block. Although, in either case, the word line layers of a block can be joined at their ends to one another so that they are commonly driven within a block, as depicted in FIG. 7B.

FIG. 5 depicts a plot of memory hole/pillar diameter in the stack of FIG. 4. The vertical axis is aligned with the stack of FIG. 4 and depicts a width (wMH), e.g., diameter, of the pillars formed by materials in the memory holes 618 and 619. In such a memory device, the memory holes which are etched through the stack have a very high aspect ratio. For example, a depth-to-diameter ratio of about 25-30 is common. The memory holes may have a circular cross-section. Due to the etching process, the memory hole and resulting pillar width can vary along the length of the hole. Typically, the diameter becomes progressively smaller from the top to the bottom of the memory hole (solid line). That is, the memory holes are tapered, narrowing at the bottom of the stack. In some cases, a slight narrowing occurs at the top of the hole near the select gate so that the diameter becomes slight wider before becoming progressively smaller from the top to the bottom of the memory hole (long dashed line). For example, the memory hole width is a maximum at the level of WL9 in the stack, in this example. The memory hole width is slightly smaller at the level of WL10, and progressively smaller at the levels of WL8 to WL0.

Due to the non-uniformity in the diameter of the memory hole and resulting pillar, the programming and erase speed of the memory cells can vary based on their position along the memory hole. With a relatively smaller diameter at the bottom of a memory hole, the electric field across the tunnel oxide is relatively stronger, so that the programming and erase speed is higher for memory cells in word lines adjacent to the relatively smaller diameter portion of the memory holes. The amount of word line coupling up and discharge is therefore relatively larger than for memory cells in word lines adjacent to the relatively larger diameter portion of the memory holes.

In another possible implementation, represented by the short dashed line, the stack is fabricated in two tiers. The bottom tier is formed first with a respective memory hole. The top tier is then formed with a respective memory hole which is aligned with the memory hole in the bottom tier. Each memory hole is tapered such that a double tapered memory hole is formed in which the width increases, then decreases and increases again, moving from the bottom of the stack to the top.

FIG. 6 depicts a close-up view of the region 622 of the stack of FIG. 4. Memory cells are formed at the different levels of the stack at the intersection of a word line layer and a memory hole. In this example, SGD transistors 680 and 681 are provided above dummy memory cells 682 and 683 and a data memory cell MC. A number of layers can be deposited along the sidewall (SW) of the memory hole 630 and/or within each word line layer, e.g., using atomic layer deposition. For example, each pillar 699 or column which is formed by the materials within a memory hole can include a charge-trapping layer 663 or film such as silicon nitride (Si₃N₄) or other nitride, a tunneling layer 664 (tunnel oxide), a channel 665 (e.g., comprising polysilicon), and a dielectric core 666. A word line layer can include a blocking oxide/block high-k material 660, a metal barrier 661, and a conductive metal 662 such as Tungsten as a control gate. For example, control gates 690, 691, 692, 693 and 694 are provided. In this example, all of the layers except the metal are provided in the memory hole. In other approaches, some of the layers can be in the control gate layer. Additional pillars are similarly formed in the different memory holes. A pillar can form a columnar active area (AA) of a NAND string.

When a memory cell is programmed, electrons are stored in a portion of the charge-trapping layer which is associated with the memory cell. These electrons are drawn into the charge-trapping layer from the channel, and through the tunneling layer. The Vth of a memory cell is increased in proportion to (e.g., with an increase in) the amount of stored charge. During an erase operation, the electrons return to the channel.

Each of the memory holes can be filled with a plurality of annular layers comprising a blocking oxide layer, a charge trapping layer, a tunneling layer and a channel layer. A core region of each of the memory holes is filled with a body material, and the plurality of annular layers are between the core region and the word line in each of the memory holes.

The NAND string can be considered to have a floating body channel because the length of the channel is not formed on a substrate. Further, the NAND string is provided by a plurality of word line layers above one another in a stack, and separated from one another by dielectric layers.

FIG. 7A depicts an example view of NAND strings in sub-blocks in a 3D configuration which is consistent with FIG. 4. Example memory cells are depicted which extend in the x direction along word lines in each sub-block. Each memory cell is depicted as a cube for simplicity. SB0 includes NAND strings 700 n, 701 n, 702 n and 703 n. SB1 includes NAND strings 710 n, 711 n, 712 n and 713 n. SB2 includes NAND strings 720 n, 721 n, 722 n and 723 n. SB3 includes NAND strings 730 n, 731 n, 732 n and 733 n. Bit lines are connected to sets of NAND strings. For example, a bit line BL0 is connected to NAND strings 700 n, 710 n, 720 n and 730 n, a bit line BL1 is connected to NAND strings 701 n, 711 n, 721 n and 731 n, a bit line BL2 is connected to NAND strings 702 n, 712 n, 722 n and 732 n, and a bit line BL3 is connected to NAND strings 703 n, 713 n, 723 n and 733 n. A sense circuit may be connected to each bit line. For example, sense circuits 400, 400 a, 400 b and 400 c are connected to bit lines BL0, BL1, BL2 and BL3, respectively. The NAND strings are examples of vertical memory strings, e.g., vertical strings, which extend upward from a substrate.

Programming and reading can occur for selected cells in one word line and one sub-block at a time. This allows each selected cell to be controlled by a respective bit line and/or source line. For example, an example set 795 of memory cells in SB0 is connected to WLL4. Similarly, the sets 796, 797 and 798 comprise data memory cells in SB1, SB2 and SB3 are connected to WLL4.

FIG. 7B depicts word line and SGD layers in an example set of blocks which is consistent with FIG. 4. Blocks BLK0, BLK1, BLK2 and BLK3 are depicted. The word line layers (WLL) in each block are depicted and long with example SGD lines. One SGD line is provided in each sub-block. BLK0 includes sub-blocks SB0, SB1, SB2 and SB3. Each circle represents a memory hole or string. The sub-blocks are elongated in the x direction and contain thousands of memory strings in practice. Additionally, many more blocks beyond those depicted are arranged in a row on the substrate. The word line layers and SGD/SGS layers may receive voltages from a row decoder 799.

FIG. 8A depicts an example Vth distribution of memory cells, where eight data states are used, in a first read condition compared to a second read condition. Eight data states are an example only as other numbers may be used such as four, sixteen or more. For the Er, A, B, C, D, E, F and G states, we have Vth distributions 820, 821, 822, 823, 824, 825, 826 and 827, respectively, in the second read condition, and 820 a, 821 a, 822 a, 823 a, 824 a, 825 a, 826 a and 827 a, respectively, in the first read condition. For the A, B, C, D, E, F and G states, we have program verify voltages VvA, VvB, VvC, VvD, VvE, VvF and VvG, respectively. Also depicted are read voltages VrAH, VrBH, VrCH, VrDH, VrEL, VrFL and VrGL, respectively, in the second read condition, and read voltages VrAL, VrBL, VrCL, VrDL, VrEH, VrFH and VrGH, respectively, in the first read condition. Also depicted is an example encoding of bits of 111, 110, 100, 000, 010, 011, 001 and 101, respectively. The bit format is: UP/MP/LP. An erase verify voltage VvEr is used during an erase operation.

This example indicates the shift in the Vth distribution for the first read condition compared to the second read condition is relatively larger when the data state is relatively lower or higher, than when the data state is mid-range. The shift may be progressively larger for progressively lower or higher data states. In one example, the read voltages of VrAL, VrBL, VrCL and VrDL are optimal for the relatively lower states of A, B, C and D, respectively, and the read voltages of VrEH, VrFH and VrGH are optimal for the relatively higher states of E, F and G, respectively, in the first read condition. Similarly, the read voltages of VrAH, VrBH, VrCH and VrDH are optimal for the relatively lower states of A, B, C and D, respectively, and the read voltages of VrEL, VrFL and VrGL are optimal for the relatively higher states of E, F and G, respectively, in the second read condition. Thus, the lower of two read voltages per state is optimal in the first read condition for the lower states and the higher of two read voltages per state is optimal in the first read condition for the higher states, in one possible implementation.

The optimum read voltages generally are midway between the Vth distributions of adjacent data states. Accordingly, as the Vth distribution shifts, the optimum read voltages shift.

The first read condition can occur when there is a long delay since a last programming or read operation. An example sequence is: program a block, wait for one hour, then read the block. The first read condition can also occur when there is a power down/power up. An example sequence is: program a block, power down/power up, then read the block. The first read condition can also occur when there is a program or read of other blocks. An example sequence is: program one block, program another block, then read the one block.

FIG. 8B depicts example bit sequences for lower, middle and upper pages of data, and associated read voltages. In this case, the memory cells each store three bits of data in one of eight data states. Example bit assignments for each state are depicted. A lower, middle or upper bit can represent data of a lower, middle or upper page, respectively. Seven programmed data states A, B, C, D, E, F and G are used in addition to the erased state, Er. With these bit sequences, the data of the lower page can be determined by reading the memory cells using read voltages (e.g., control gate or word line voltages) of VrA and VrE. The lower page (LP) bit=1 if Vth<=VrA or Vth>VrE. LP=0 if VrA<Vth<=VrE. Generally, a memory cell can be sensed by sense circuitry while a read voltage is applied. If the memory cell is in a conductive state at a sense time, its threshold voltage (Vth) is less than the read voltage. If the memory cell is in a non-conductive state, its Vth is greater than the read voltage.

The read voltages which are used to read a page of data are determined by transitions from 0 to 1 or 1 to 0 in the encoded bits (code word) for each state. For example, the LP bit transitions from 1 to 0 between Er and A, and from 0 to 1 between D and E. Accordingly, the read voltages for the LP are VrA and VrE.

The data of the middle page can be determined by reading the memory cells using read voltages VrB, VrD and VrF. The middle page (MP) bit=1 if Vth<=VrB or VrD<Vth<=VrF. MP=0 if VrB<Vth<=VrD or Vth>VrF. For example, the MP bit transitions from 1 to 0 between A and B, from 0 to 1 between C and D, and from 1 to 0 between E and F. Accordingly, the read voltages for the MP are VrB, VrD and VrF.

The data of the upper page can be determined by reading the memory cells using read voltages of VrC and VrG. The upper page (UP) bit=1 if Vth<=VrC or Vth>VrG. UP=0 if VrC<Vth<=VrG. For example, the UP bit transitions from 1 to 0 between B and C, and from 0 to 1 between F and G. Accordingly, the read voltages for the UP are VrC and VrG. The read voltages are depicted as VrA, VrB, VrC, VrD, VrE, VrF and VrG, where each of these can represent the first or second read values, whichever is optimal.

FIG. 9 depicts a waveform of an example programming operation. The horizontal axis depicts a program loop (PL) number and the vertical axis depicts control gate or word line voltage. Generally, a programming operation can involve applying a pulse train to a selected word line, where the pulse train includes multiple program loops or program-verify iterations. The program portion of the program-verify iteration comprises a program voltage, and the verify portion of the program-verify iteration comprises one or more verify voltages.

Each program voltage includes two steps, in one approach. Further, Incremental Step Pulse Programming (ISPP) is used in this example, in which the program voltage steps up in each successive program loop using a fixed or varying step size. This example uses ISPP in a single programming pass in which the programming is completed. ISPP can also be used in each programming pass of a multi-pass operation.

The waveform 900 includes a series of program voltages 901, 902, 903, 904, 905, . . . 906 that are applied to a word line selected for programming and to an associated set of non-volatile memory cells. One or more verify voltages can be provided after each program voltage as an example, based on the target data states which are being verified. 0 V may be applied to the selected word line between the program and verify voltages. For example, A- and B-state verify voltages of VvA and VvB, respectively, (waveform 910) may be applied after each of the program voltages 901 and 902. A-, B- and C-state verify voltages of VvA, VvB and VvC (waveform 911) may be applied after each of the program voltages 903 and 904. After several additional program loops, not shown, E-, F- and G-state verify voltages of VvE, VvF and VvG (waveform 912) may be applied after the final program voltage 906.

FIG. 10A depicts a plot of example waveforms in a programming operation, showing coupling up of a word line voltage. The time period shown represents one program-verify iteration. The horizontal axis depicts time and the vertical axis depicts word line voltage, Vwl. A program voltage 1000 is applied to a selected word line from t0-t4 and reaches a magnitude of Vpgm. The program voltage can temporarily pause at an intermediate level such as Vpass to avoid a single large transition which can have undesired coupling effects. A pass voltage 1005 is applied to the unselected word lines from t0-t19 and reaches a magnitude of Vpass, which is sufficiently high to provide the cells in a conductive state so that the sensing (e.g., verify) operations can occur for the cells of the selected word line. The pass voltage includes an increasing portion, a fixed amplitude portion, for instance, at Vpass and a decreasing portion. Optionally, the pass voltage may be increased sooner relative to the program voltage so that Vpass is reached by to.

A verify voltage 1010 is applied to the selected word line. In this example, all seven verify voltages are applied, one after another. An eight-level memory device is used in this example. Verify voltages of VvA, VvB, VvC, VvD, VvE, VvF and VvG are applied at t8, t9, t10, t11, t12, t13 and t14, respectively. The sense circuits may be activated during each verify voltage. The waveform decreases from VvG to 0 V or other steady state level from t15-t16.

For the unselected word lines, the decrease in Vpass will cause the cells to transition from a conductive state to a non-conductive state. In particular, when the Vpass falls below a cutoff level, Vcutoff (the dotted line at t18), the channel of the cell will become cutoff, e.g., the cell will become non-conductive. When a cell becomes non-conductive, it acts as a capacitor in which the control gate is one plate and the channel is another plate. A cell becomes non-conductive when Vcg<Vcutoff or Vcg<(Vth+Vsl), where Vcg is the control gate voltage of the cell (the word line voltage), Vth is the threshold voltage of the cell and Vsl is the source line voltage which in turn is approximately the voltage at the source terminal of the cell. For a cell in the highest programmed state, e.g., the G state, the Vth can be as low as VvG (or lower due to post-programming charge loss) and as high as the Vth at the upper tail of the G state in the Vth distribution 827 or 827 a in FIG. 8A. Vcutoff can therefore be as low as VvG+Vsl or as high as Vth of G state upper tail+Vsl. As the pass voltage 1005 decreases from Vcutoff to 0 V, the channel is capacitively coupled down by a similar amount, as represented by plot 1015 in FIG. 10B.

The voltage swing while the channel is cutoff will be larger when Vsl is larger. However, since Vch=Vsl, the minimum downcoupled level of Vch will be essentially independent of Vsl. For example, a 6 V swing in the word line voltage (e.g., Vcutoff=6 V) with Vsl=1 V will result in about the same minimum downcoupled level of Vch as a 5 V swing in the word line voltage (e.g., Vcutoff=5 V) with Vsl=0 V.

The plot 1012 represents the coupling up of the word line voltages from t19-t20. The coupling up is depicted as occurring relatively quickly but this is not to scale. In practice, the verify operation, e.g., from t5-t19, may consume about 100 microseconds, while the coupling up of the word line may be significantly longer, in the millisecond range such as 10 milliseconds.

FIG. 10B depicts a plot of a channel voltage (Vch) corresponding to FIG. 10A. For an unselected memory string (a string not having a cell which is programmed in the current program loop), Vch will be boosted to a level such as 8 V (not shown) during the program voltage, e.g., from t0-t4. This boosting is achieved by providing the SGD and SGS transistors of the unselected string in a non-conductive state to cause Vch to float. Vch is coupled higher due to capacitive coupling when Vpass and Vpgm are applied to the word lines. For a selected memory string (a string having a cell which is programmed in the current program loop), Vch is typically grounded as shown during the program voltage.

During the verify voltages, Vch may be initially at about 1 V, for instance, for a selected memory string. Vch is about the same as Vsl for the channels of the selected memory strings. Vsl is set based on a type of sensing which is used. Examples include negative sensing in which Vsl is about 1 V and positive sensing in which Vsl is about 0 V and a negative word line voltage is used. The techniques described herein apply regardless of the level of Vsl or the type of sensing used.

The channel is capacitively coupled down to a minimum level from t18-t19 and then begins to return to a final level of, e.g., 0 V from t19-t20. If the voltages of the word lines are allowed to float starting at t19, the voltages (plot 1012) are capacitively coupled higher by the increase in Vch. The voltages of the word lines float to a peak level of Vwl_coupled_up, thereby reaching the second read condition. For example, Vcutoff may be 6 V, so that there is a 6 V change in the word line voltage, e.g., 6-0 V, which is coupled to the channel. With the initial value of Vch at 1 V and a 90% coupling ratio, the minimum Vch may be about 1−6×0.9=−4.4 V, for instance. Accordingly, there is a 4.4 V increase in Vch which is coupled to the word line, e.g., control gate, of the cells. Vwl_coupled_up may be about 4.4×0.9=4 V. The voltages of the word lines are floated by disconnected the word lines from a word line driver.

FIG. 10C depicts a plot of example waveforms in a read operation, showing coupling up of a word line voltage. A read operation is similar to a verify operation as both are sensing operations and both can provide a coupling up of the word line voltages. The horizontal axis depicts time and the vertical axis depicts word line voltage, Vwl. Pass voltages 1115, 1116 and 1117 are applied to the unselected word lines from t0-t3, t4-t8 and t9-t12, respectively, and have a magnitude of Vpass. The pass voltage includes an increasing portion, a portion at Vpass and a decreasing portion. A read voltage includes separate waveforms 1120 (at levels of VrAH and VrEL), 1121 (at levels of VrBH, VrDH and VrFL) and 1122 (at levels of VrCH and VrGL) for each of the lower, middle and upper pages, respectively, consistent with FIGS. 8A and 8B. The read voltages are optimized for the second read condition, as an example, and are applied to the selected word line. An eight-level memory device is used in this example.

For the unselected word lines, the decrease in Vpass will cause the cells to transition from a conductive state to a non-conductive state, as discussed. The dotted line at t13 indicates when a G state cell becomes non-conductive. As the pass voltage 1117 decreases from Vcutoff to 0 V, the channel is capacitively coupled down by a similar amount, as represented by the plot 1035 in FIG. 10D. As the channel voltage increases after t14, the word line voltages are floated and are coupled higher, to Vwl_coupled_up.

FIG. 10D depicts a plot of a channel voltage (Vch) corresponding to FIG. 10C. The channel is capacitively coupled down to a minimum level of Vch_min from t13-t14 and then begins to return to a final level of, e.g., 0 V from t14-t15. If the voltages of the word lines are allowed to float starting at t14, the voltages (plot 1032) are capacitively coupled higher by the increase in Vch (plot 1035). The voltages of the word lines float to a peak level of Vwl_coupled_up, as discussed.

FIG. 10E depicts the waveforms of FIG. 10C showing a decay of the coupled up voltage of the word line. The time scale is different than in FIGS. 10A-10D and represents a longer time period such as one or more hours. The plot 1123 depicts the read voltages (corresponding to the waveforms 1120-1122 in FIG. 10C) in a time period t041. The plot 1123 a depicts the pass voltages (corresponding to the waveforms 1115-1117 in FIG. 10C). A plot 1125 depicts an increase in Vwl to a coupled up level (Vwl_coupled_up) due to coupling (in a time period t1-t2) followed by a decay of Vwl in a time period t2-t3. Generally, the increase in Vwl occurs relatively quickly compared to the time period of the decay.

FIG. 10F depicts a plot of a channel voltage consistent with FIG. 10E. A decrease is followed by an increase (plot 1126) in the time period t1-t2. Vch is about 0 V from t2-t3 (plot 1127).

FIG. 10G depicts a plot of a Vth of a memory cell connected to the coupled up word line, consistent with FIGS. 10E and 10F. For a cell in an example data state, such as the A state, the Vth is at an initial level, Vth_initial, from t0-t1. This represents the first read condition. Vth increases from t1-t2 (plot 1128) due to coupling at the same time as the increase in Vch, to a peak level of Vth_coupled_up. This represents the second read condition. The Vth then gradually decreases back to Vth_initial from t1-t3.

FIG. 11A depicts control gate and channel voltages on a memory cell which acts as a capacitor when the control gate voltage is decreased in a sensing operation. The first read issue is caused by the stacking of word line planes or layers in 3D, where the channels of the memory cells are floating and not coupled to the substrate as in 2D Flash NAND architectures. Word line coupling and electron trapping in oxide-nitride-oxide (ONO) layers are the sources of the first read issue.

As discussed, after a read/verify operation, when the read pass voltage (Vpass) applied on a word line ramps down, the G-state cells, for example, with Vth of 5 V, cut off the channel when Vpass reduces to 5 V. The floating channel potential then is pushed down to a negative value when Vpass is further reduced to Vss. Next, the negative voltage in the channel shown above (about −4.5 V) increases after the read operation finishes by attracting positive charges. Since the data word lines are floating, the amount of holes needed to charge up the channel is relatively small, so the selected and unselected word lines can be quickly coupled up to around 4 V (assuming a coupling ratio of 90%). The potential on the word lines remains at about 4 V for a while. This attracts and traps electron in the tunnel ONO layers and causes a Vth upshift or downshift for the lower or higher data states, respectively. The word line voltage thus rises to about 4 V after the read operation due to the coupling of the word line to the floating channel potential.

The top plate represents the control gate or word line and the bottom plate represents the channel. The capacitor 1040 represents a memory cell when the word line voltage decreases from 8 V (Vpass) to 5 V (Vcutoff such as VvG or slightly higher) and Vch=0 V. The capacitor 1042 represents a memory cell when the word line voltage reaches 0 V, so that Vch is down coupled to about −4.5 V. The capacitor 1044 represents a memory cell when the associated word line voltage starts to float. The capacitor 1046 represents a memory cell when the associated word line voltage reaches Vwl_coupled_up in the second read condition. If the Vth of the memory cell is less than 4 V (e.g., the cell is in the erased state or lower programmed state), the memory cell will be weakly programmed so that its Vth increases. If the Vth of the memory cell is more than 4 V (e.g., the cell is in a higher programmed state), the memory cell will be weakly erased so that its Vth decreases. The capacitor 1048 represents a memory cell after a significant amount of time has passed, e.g., an hour or more such that the word line has discharged to the first read condition.

When a data word line voltage floats, the amount of holes needed to charge up the channel is relatively small. As a result, the selected word line can be relatively quickly coupled up to about 4 V, for example. The potential on the selected word line remains at about 4 V for a while, attracting electrons trapped in the tunnel oxide-nitride-oxide (ONO) layers and causing a Vth up-shift. If the wait before the next read operation is long enough, the coupled up potential of the word line will be discharged, and the trapped electrons will be de-trapped. The first read condition will occur again.

FIG. 11B depicts a portion of the memory cell MC of FIG. 6 showing electron injection into a charge trapping region during weak programming. The memory cell includes a control gate 694, a metal barrier 661 a, a blocking oxide 660 a, a charge-trapping layer 663, a tunneling layer 664, a channel 665 and a dielectric core 666. Due to the elevated word line voltage, an electric field (E) is created which attracts electrons (see example electron 1050) into the charge trapping layer, increasing the Vth. This weak programming may be caused by the Poole-Frenkel effect, in which an electrical insulator can conduct electricity. This is a kind of electron tunneling through traps. A weak erasing similarly involves an electric field which repels electrons from the charge trapping layer, decreasing the Vth.

As discussed, the structure of the memory apparatus (e.g., BiCS) can result in a block being in the second read condition or state right after a read operation is performed (the word lines have a residual potential of ˜4V). The duration for which the block remains in second read condition before the word line potential discharges or drops to 0V depends on the ambient temperature. For example, the time to discharge can be approximately 2 minutes at 85 degrees Celsius, 2-3 hours at 55 degrees Celsius, and ˜15 hours at 25 degrees Celsius. Data retention following write/erase (WE) cycles is a critical limiter affecting reliability as ON pitch scaling in memory devices, such as NAND, continues to drive down costs. This issue affects quad level cells (QLC) much more with intrinsically lower margins and higher programmed states. Data retention is better during bake (i.e., high temperature) if the block is kept in the second read condition. The word line potential prevents electron loss through tunnel oxide on high threshold voltage Vt states. For lower states having fewer electrons in the charge trapping layer (CTL), it is expected that in the second read condition, the threshold voltage Vth would be higher due to the word line voltage creep up (˜4V) which attracts electrons at the tunneling layer (TNL) and poly-Si interface. On the other hand, at higher states, electrons trapped in the CTL gets trapped at the CTL and Blk Ox interface which then results to lower threshold voltage Vth. Effective threshold voltages Vth shift as shown in FIG. 8A, for example. Since verify is done while the memory cells are in second read condition, read levels are optimized for this condition.

Consequently, described herein is a memory apparatus (e.g., memory device 100 in FIG. 1A) including a plurality of memory cells (e.g., memory cell 200 in FIG. 1B). Each of the plurality of memory cells is connected to one of a plurality of word lines (e.g., WLL0-WLL10 in FIG. 4) and arranged in one of a plurality of blocks (e.g., BLK0, BLK1, BLK2 and BLK3 of FIG. 7B) and configured to retain a threshold voltage Vth corresponding to one of a plurality of data states (e.g., “Er”, “A”, “B”, “C”, “D”, “E”, “F” and “G” in FIG. 8A). As discussed above, the plurality of memory cells are operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up (e.g., plot 1012 of FIG. 10A) to a residual voltage level (e.g., Vwl_coupled_up). The apparatus also includes a control circuit (e.g., control circuitry 110, controller 122, row decoder 124, source control circuits 127, read/write circuits 128, sense blocks 51, 52, 53, and column decoder 132 in FIG. 1A) coupled to the plurality of word lines and configured to determine a power on event of the apparatus in which the plurality of memory cells are in the first read condition (a power on reset (POR) moves blocks to the first read condition). The control circuit is additionally configured to periodically apply a predetermined refresh read voltage to a selected one of the plurality of word lines for a predetermined period of time (i.e., a read refresh) for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the apparatus. In other words, the fixed voltage (i.e., the predetermined refresh read voltage) is applied to all word lines for the predetermined period of time (e.g., ˜1 milliseconds) on a large group of blocks (1 to 128) to keep the blocks in second read condition. This operation cycles through all groups of blocks to cover the whole die. Such a read refresh can improve power on data retention.

According to an aspect, the at least one data retention factor includes a die temperature of the plurality of memory cells, since temperature can affect the amount of time it takes for the residual potential of the word lines to discharge. As discussed above, the apparatus includes a temperature sensor 115 (FIG. 1A) in communication with the control circuit and configured to measure the die temperature of the plurality of memory cells. As best shown in FIG. 12, the apparatus also includes a refresh frequency and temperature lookup table including the plurality of specified intervals each corresponding to one of a plurality of magnitudes of the die temperature. Since data retention is worse at higher temperature compared to low temperature, and the discharge of word line potential to the first read condition tracks the same trend, the refresh operation is more frequent at 85 degrees Celsius (every e.g., 2 minutes), compared to 55 degrees Celsius (e.g., every 2 hours) or 25 degrees Celsius (e.g., every 10 hours), according to an aspect. Thus, the control circuit is further configured to determine the die temperature of the plurality of memory cells using the temperature sensor. Alternatively, a temp-code feature in NAND can be used to detect the temperature and increase refresh frequency (shorter one of the plurality of specified intervals) at higher temperatures. The control circuit is then configured to determine and utilize one of the plurality of specified intervals based on the refresh frequency and temperature lookup table. So, for the read refresh the frequency of refresh can be controlled in NAND or by firmware on the apparatus, and can be made to depend on criterion like ambient temperature.

Alternatively, according to an aspect, the at least one data retention factor includes a failure bit count associated with reading at least a subset of the plurality of memory cells. As discussed above, the control circuit further includes an error-correction code engine 245 (FIG. 1A) configured to determine and correct errors in reading the plurality of memory cells. So, the control circuit is further configured to read the at least the subset of the plurality of memory cells and determine the failure bit count for the at least the subset of the plurality of memory cells using the error-correction code engine 245. The control circuit is also configured to determine whether the failure bit count exceeds a predetermined failure bit count threshold. The control circuit is then configured to apply the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the failure bit count exceeding the predetermined failure bit count threshold. So, the failure bit count or error count can trigger the application of the predetermined refresh read voltage to maintain the plurality of memory cells of the plurality of blocks in the second read condition.

Now referring to FIGS. 13A and 13B, the threshold voltage Vth possible for each of the plurality of memory cells spans a threshold window 1200 and each of the plurality of memory cells can be configured to store a plurality of bits. According to an aspect and as shown, the plurality of bits can include four bits (i.e., quad level cell (QLC)), so the plurality of data states includes an erased state at a first end 1202 of the threshold window (e.g., S0 state of FIG. 13A) and a plurality of programmed data states each corresponding to the threshold voltage Vth being higher than the threshold voltage associated with the erased state S0. The plurality of programmed data states include one or more higher data states at a second end 1204 of the threshold window opposite the first end 1202 (e.g., S14 and S15 states of FIG. 13A) and associated with the threshold voltage Vth being higher than the threshold voltage associated with the erased state and at least one other of the plurality of programmed data states (e.g., S1-S13 states of FIG. 13A). As discussed above and referring now to FIG. 13B, the data stored in the plurality of memory cells can be stored in a plurality of pages respectively associated with each the plurality of bits stored by each of the plurality of memory cells and encoded according to an encoding scheme of the plurality of data states. Accordingly, the at least the subset of the plurality of memory cells includes at least one of the plurality of pages. So, the control circuit is further configured to select the at least one of the plurality of pages based on which of the at least one of the plurality of pages is associated with the one or more higher data states. The control circuit is also configured to read the data stored in the at least one of the plurality of pages. In addition, the control circuit is configured to determine the failure bit count for the at least one of the plurality of pages using the error-correction code engine 245. While the refresh read discussed herein is primarily discussed for triple level cells (TLC) or three bits per memory cells and QLC die, it should be appreciated that the plurality of memory cells can be configured to store any numbers of bits.

Still referring to FIGS. 13A and 13B, instead of the data states shown in FIG. 8A, the plurality of programmed data states includes, in order of the threshold voltage Vth increasing in magnitude, a first data state S1 and a second data state S2 and a third data state S3 and a fourth data state S4 and a fifth data state S5 and a sixth data state S6 and a seventh data state S7 and an eighth data state S8 and a ninth data state S9 and a tenth data state S10 and an eleventh data state S11 and a twelfth data state S12 and a thirteenth data state S13 and a fourteenth data state S14 and a fifteenth data state S15. The plurality of pages includes a top page, an upper page, a middle page, and a lower page respectively associated with each the four bits stored by each of the plurality of memory cells and encoded according to the encoding scheme of the plurality of data states. So, according to an aspect, the control circuit is further configured to apply a sequence of read voltages corresponding to the fifth data state S5, the tenth data state S10, the twelfth data state S12, and the fifteenth data state S15 to the selected one of the plurality of word lines to read the data of the top page. It should be understood that, depending on the number of bits stored in each of the plurality of memory cells and encoding, other data states may be read instead. The control circuit is also configured to determine the failure bit count for the data stored in the top page using the error-correction code engine 245. In other words, the control circuit can keep track of the failure bit count for the top page and trigger refresh operations when failure bit count is larger than a set threshold bit count. So, failure bit count degradation on pages containing higher states (S15, S14, etc.) can be used to track data retention. In the example shown here, a read of the data in the top page includes a read of the fifteenth data state S15 (S14-S15 valley).

As an alternative using to the failure bit count for the data stored in one of the plurality of pages to trigger the application of the predetermined refresh read voltage to maintain the plurality of memory cells of the plurality of blocks in the second read condition, the at least one data retention factor can include a delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage between the one or more higher data states. Thus, the control circuit is further configured to read the at least the subset of the plurality of memory cells using a default read voltage associated with the one or more higher data states and using an adjusted read voltage being less than the default read voltage by a predetermined delta control gate voltage. The control circuit is also configured to determine the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage. The control circuit is configured to determine whether the delta bit count quantity exceeds a predetermined delta bit count quantity threshold. The control circuit then applies the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time (i.e., refresh read) for each of the plurality of blocks in response to the delta bit count quantity exceeding the predetermined delta bit count quantity threshold. So, if the bit count between the two read levels (default read voltage and adjusted read voltage) is larger than the predetermined delta bit count quantity threshold, this can be used to indicate that data retention shift has occurred.

One technique that can be used for tracking a data retention amount in NAND is to perform reads of the fifteenth data state S15 at two different voltages (e.g., default read voltage and default read voltage minus a predetermined delta control gate voltage) and count the number of bits between the two levels. According to an aspect, such data retention tracking can be completed without the controller. The number will be large post data retention, and a value larger than a specific bit count or predetermined delta bit count quantity threshold can be used to trigger refresh reads. Again, the plurality of bits can include four bits and the plurality of programmed data states can include, in order of the threshold voltage increasing in magnitude, the first data state S1 and the second data state S2 and the third data state S3 and the fourth data state S4 and the fifth data state S5 and the sixth data state S6 and the seventh data state S7 and the eighth data state S8 and the ninth data state S9 and the tenth data state S10 and the eleventh data state S11 and the twelfth data state S12 and the thirteenth data state S13 and the fourteenth data state S14 and the fifteenth data state S15. FIG. 14 shows distributions of the threshold voltage Vth of the fourteenth data state and a fifteenth data state before and after a check of data retention.

So, the control circuit is further configured to read the at least the subset of the plurality of memory cells using the default read voltage (indicated as Read level in FIG. 14) associated with the fifteenth data state and using the adjusted read voltage (indicated as Read level—DVCG in FIG. 14) being less than the default read voltage by the predetermined delta control gate voltage (i.e., DVCG) to determine the delta bit count quantity (indicated as Bit count in FIG. 14) of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage. As shown in FIG. 14, before data retention, the delta bit count quantity is only a small number of bits. However, after data retention, the delta bit count quantity is a large number of bits. According to an aspect, the at least the subset of the plurality of memory cells are ones of the plurality of memory cells connected to a selected one of the plurality of word lines. In addition, the predetermined delta control gate voltage DVCG can be approximately 100 millivolts.

Furthermore, the control circuit is further configured to determine whether a user operation of the apparatus has been initiated. The control circuit is configured to interrupt periodically applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks at the one of the plurality of specified intervals based on the at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the user operation of the apparatus has been initiated.

Now referring to FIG. 15, a method of operating a memory apparatus is also provided. As discussed above, the memory apparatus includes a plurality of memory cells. Each of the plurality of memory cells connected to one of a plurality of word lines and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to one of a plurality of data states. The plurality of memory cells are operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up to a residual voltage level. So, the method includes the step of 1300 determining a power on event of the memory apparatus in which the plurality of memory cells are in the first read condition (POR (power on reset) causes the block to go to the first read condition). The method continues with the step of 1302 periodically applying a predetermined refresh read voltage to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the memory apparatus. In other words, a read refresh on whole die is performed after the power on reset to set the second read condition. The refresh operation is expected to take tens of milliseconds for the whole die.

Again, the at least one data retention factor includes a die temperature of the plurality of memory cells and the memory apparatus further includes a temperature sensor configured to measure the die temperature of the plurality of memory cells. The apparatus also includes a refresh frequency and temperature lookup table including the plurality of specified intervals each corresponding to one of a plurality of magnitudes of the die temperature (FIG. 12). So, the step of 1302 periodically applying a predetermined refresh read voltage to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the memory apparatus can include the step of 1304 performing an initial refresh read. The method further includes the step of 1306 determining the die temperature of the plurality of memory cells using the temperature sensor. As an alternative, the method can include reading temp-code to determine die temperature. The next step of the method is 1308 determining and utilizing one of the plurality of specified intervals based on the refresh frequency and temperature lookup table. So, a refresh frequency can be determined based on the look-up table (e.g., 2 min to 10 hour).

Alternatively, according to an aspect, the at least one data retention factor includes the failure bit count associated with reading the at least the subset of the plurality of memory cells. So, the step of 1302 periodically applying a predetermined refresh read voltage to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the memory apparatus can include the step of 1310 reading the at least the subset of the plurality of memory cells. The method can continue with the step of 1312 determining the failure bit count for the at least the subset of the plurality of memory cells using the error-correction code engine. Next, 1314 determining whether the failure bit count exceeds a predetermined failure bit count threshold. The method then includes the step of 1316 applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the failure bit count exceeding the predetermined failure bit count threshold.

As previously discussed, the threshold voltage Vth possible for each of the plurality of memory cells spans a threshold window 1200 (FIG. 13A) and each of the plurality of memory cells can be configured to store a plurality of bits. According to an aspect and as shown, the plurality of bits can include four bits, so the plurality of data states includes an erased state at a first end 1202 (FIG. 13A) of the threshold window (e.g., S0 state of FIG. 13A) and a plurality of programmed data states each corresponding to the threshold voltage being higher than the threshold voltage associated with the erased state S0 (FIG. 13A). The plurality of programmed data states include one or more higher data states at a second end 1204 of the threshold window opposite the first end 1202 (e.g., S14 and S15 states of FIG. 13A) and associated with the threshold voltage being higher than the threshold voltage associated with the erased state and at least one other of the plurality of programmed data states (e.g., S1-S13 states of FIG. 13A). Again, a data stored in the plurality of memory cells is stored in a plurality of pages respectively associated with each the plurality of bits stored by each of the plurality of memory cells and encoded according to an encoding scheme of the plurality of data states. The at least the subset of the plurality of memory cells includes at least one of the plurality of pages. So, the method further includes the step of selecting the at least one of the plurality of pages based on which of the at least one of the plurality of pages is associated with the one or more higher data states. The next step of the method is reading the data stored in the at least one of the plurality of pages. The method continues with the step of determining the failure bit count for the at least one of the plurality of pages using the error-correction code engine.

The plurality of bits includes four bits, the plurality of programmed data states includes, in order of the threshold voltage increasing in magnitude, a first data state S1 and a second data state S2 and a third data state S3 and a fourth data state S4 and a fifth data state S5 and a sixth data state S6 and a seventh data state S7 and an eighth data state S8 and a ninth data state S9 and a tenth data state S10 and an eleventh data state S11 and a twelfth data state S12 and a thirteenth data state S13 and a fourteenth data state S14 and a fifteenth data state S15. The plurality of pages includes a top page, an upper page, a middle page, and a lower page respectively associated with each the four bits stored by each of the plurality of memory cells and encoded according to the encoding scheme of the plurality of data states. So, the method further includes the step of applying a sequence of read voltages corresponding to the fifth data state S5, the tenth data state S10, the twelfth data state S12, and the fifteenth data state S15 to the selected one of the plurality of word lines to read the data of the top page. The next step of the method is determining the failure bit count for the data stored in the top page using the error-correction code engine.

Again, a delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage between the one or more higher data states can be used to trigger the application of the predetermined refresh read voltage to maintain the plurality of memory cells of the plurality of blocks in the second read condition. Thus, the method further includes the step of reading the at least the subset of the plurality of memory cells using a default read voltage associated with the one or more higher data states and using an adjusted read voltage being less than the default read voltage by a predetermined delta control gate voltage. The method continues with the step of determining the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage. Next, determining whether the delta bit count quantity exceeds a predetermined delta bit count quantity threshold. The method also includes the step of applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the delta bit count quantity exceeding the predetermined delta bit count quantity threshold.

As discussed above, a technique that can be used for tracking a data retention amount in NAND is to perform reads of the fifteenth data state S15 at two different voltages (e.g., default read voltage and default read voltage minus a predetermined delta control gate voltage) and count the number of bits between the two levels. Thus, the method further includes the step of reading the at least the subset of the plurality of memory cells using the default read voltage associated with the fifteenth data state and using the adjusted read voltage being less than the default read voltage by the predetermined delta control gate voltage to determine the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage.

In addition, the method further includes the step of 1318 determining whether a user operation of the memory apparatus has been initiated. The next step of the method is 1320 interrupting periodically applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks at the one of the plurality of specified intervals based on the at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the user operation of the memory apparatus has been initiated. So, the refresh can be interrupted at any time for user operations (e.g., read, write, etc.).

FIGS. 16A-16B show a sector fail rate (SFR) versus failure bit count (FBC)/sector for varying data retention periods of time with a refresh or dummy read (FIG. 16B) and without the refresh read (FIG. 16A) for an example memory apparatus. For the data shown in FIG. 16B, the refresh or dummy read is completed on WL95 string 0 LP every 60 seconds. As shown, data retention data was collected with the die in a power-on condition and 85 degrees Celsius bake. Failure bit count after data retention is ˜10% better when blocks are kept in second read condition (i.e., word lines at ˜4V). Consequently, data retention capability and endurance can be extended on by utilizing the read refresh disclosed herein.

FIG. 17 shows threshold voltage Vth distributions for an example memory apparatus after 80 hour data retention 85 degrees Celsius bake with 2000 write/erase cycles with and without the refresh or dummy read. As shown, the lower tail for the twelfth data state S12, thirteenth data state S13, fourteenth data state S14, and fifteenth data state S15 is significantly better with the refresh read.

FIG. 18 shows a threshold voltage margin improvement for data retention with the refresh read for an example memory apparatus. Specifically, the memory cells of the example memory apparatus each store four bits (i.e., QLC) and the threshold margin is measured after an hour data retention bake at 85 degrees Celsius for 1, 100, 500, 1000, and 2000 write/erase cycles. The threshold voltage Vth margin improves by ˜750 mv for 2000 write/erase cycles after the 80 hr data retention bake at 85 degrees Celsius.

FIG. 19 shows threshold voltage distributions for an example memory apparatus after 80 hour data retention 85 degrees Celsius bake with various write/erase cycles with and without the refresh or dummy read. As shown, the lower tail for the twelfth data state S12, thirteenth data state S13, fourteenth data state S14, and fifteenth data state S15 are significantly better with read refresh. The higher word line potential resulting from the refresh read disclosed herein prevents electron loss through tunnel oxide (especially post write/erase cycling) improving the lower tails post data retention.

Clearly, changes may be made to what is described and illustrated herein without, however, departing from the scope defined in the accompanying claims. The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” “top”, “bottom”, and the like, may be used herein for ease of description to describe one element's or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptions used herein interpreted accordingly. 

What is claimed is:
 1. An apparatus, comprising: a plurality of memory cells of the non-volatile type, each of the plurality of memory cells connected to one of a plurality of word lines and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to one of a plurality of data states, the plurality of memory cells operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up to a residual voltage level; and a control circuit coupled to the plurality of word lines and configured to: determine a power on event of the apparatus in which a voltage of 0 V or less is applied to the word lines, thereby causing the plurality of memory cells are in the first read condition, periodically apply a predetermined refresh read voltage, which is less than a programming voltage, to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the apparatus, and wherein the application of the refresh read voltage is based on at least one of a die temperature of a die of the apparatus and a failure bit count exceeding a predetermined failure bit count threshold.
 2. The apparatus as set forth in claim 1, wherein the at least one data retention factor includes the die temperature, the apparatus further includes a temperature sensor in communication with the control circuit and configured to measure the die temperature of the plurality of memory cells and a refresh frequency and temperature lookup table including the plurality of specified intervals each corresponding to one of a plurality of magnitudes of the die temperature, and the control circuit is further configured to: determine the die temperature of the plurality of memory cells using the temperature sensor; and determine and utilize one of the plurality of specified intervals based on the refresh frequency and temperature lookup table.
 3. The apparatus as set forth in claim 1, wherein the control circuit further includes an error-correction code engine configured to determine and correct errors in reading the plurality of memory cells, the at least one data retention factor includes the failure bit count associated with reading at least a subset of the plurality of memory cells, and the control circuit is further configured to: read the at least the subset of the plurality of memory cells; determine the failure bit count for the at least the subset of the plurality of memory cells using the error-correction code engine; determine whether the failure bit count exceeds a predetermined failure bit count threshold; and apply the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the failure bit count exceeding the predetermined failure bit count threshold.
 4. The apparatus as set forth in claim 3, wherein the threshold voltage possible for each of the plurality of memory cells spans a threshold window, each of the plurality of memory cells is configured to store a plurality of bits, the plurality of data states includes an erased state at a first end of the threshold window and a plurality of programmed data states each corresponding to the threshold voltage being higher than the threshold voltage associated with the erased state, the plurality of programmed data states including one or more higher data states at a second end of the threshold window opposite the first end and associated with the threshold voltage being higher than the threshold voltage associated with the erased state and at least one other of the plurality of programmed data states, a data stored in the plurality of memory cells is stored in a plurality of pages respectively associated with each the plurality of bits stored by each of the plurality of memory cells and encoded according to an encoding scheme of the plurality of data states, the at least the subset of the plurality of memory cells includes at least one of the plurality of pages, and wherein the control circuit is further configured to: select the at least one of the plurality of pages based on which of the at least one of the plurality of pages is associated with the one or more higher data states; read the data stored in the at least one of the plurality of pages; and determine the failure bit count for the at least one of the plurality of pages using the error-correction code engine.
 5. The apparatus as set forth in claim 4, wherein the plurality of bits includes four bits, the plurality of programmed data states includes, in order of the threshold voltage increasing in magnitude, a first data state and a second data state and a third data state and a fourth data state and a fifth data state and a sixth data state and a seventh data state and an eighth data state and a ninth data state and a tenth data state and an eleventh data state and a twelfth data state and a thirteenth data state and a fourteenth data state and a fifteenth data state, the plurality of pages includes a top page, an upper page, a middle page, and a lower page respectively associated with each the four bits stored by each of the plurality of memory cells and encoded according to the encoding scheme of the plurality of data states, and wherein the control circuit is further configured to: apply a sequence of read voltages corresponding to the fifth data state, the tenth data state, the twelfth data state, and the fifteenth data state to the selected one of the plurality of word lines to read the data of the top page; and determine the failure bit count for the data stored in the top page using the error-correction code engine.
 6. The apparatus as set forth in claim 3, wherein the threshold voltage possible for each of the plurality of memory cells spans a threshold window, each of the plurality of memory cells is configured to store a plurality of bits, the plurality of data states includes an erased state at a first end of the threshold window and a plurality of programmed data states each corresponding to the threshold voltage being higher than the threshold voltage associated with the erased state, the plurality of programmed data states including one or more higher data states at a second end of the threshold window opposite the first end and associated with the threshold voltage being higher than the than the threshold voltage associated with the erased state and at least one other of the plurality of programmed data states, the at least one data retention factor includes a delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage between the one or more higher data states, and wherein the control circuit is further configured to: read the at least the subset of the plurality of memory cells using a default read voltage associated with the one or more higher data states and using an adjusted read voltage being less than the default read voltage by a predetermined delta control gate voltage; determine the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage; determine whether the delta bit count quantity exceeds a predetermined delta bit count quantity threshold; and apply the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the delta bit count quantity exceeding the predetermined delta bit count quantity threshold.
 7. The apparatus as set forth in claim 6, wherein the plurality of bits includes four bits, the plurality of programmed data states includes, in order of the threshold voltage increasing in magnitude, a first data state and a second data state and a third data state and a fourth data state and a fifth data state and a sixth data state and a seventh data state and an eighth data state and a ninth data state and a tenth data state and an eleventh data state and a twelfth data state and a thirteenth data state and a fourteenth data state and a fifteenth data state, and wherein the control circuit is further configured to read the at least the subset of the plurality of memory cells using the default read voltage associated with the fifteenth data state and using the adjusted read voltage being less than the default read voltage by the predetermined delta control gate voltage to determine the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage.
 8. The apparatus as set forth in claim 1, wherein the control circuit is further configured to: determine whether a user operation of the apparatus has been initiated, and interrupt periodically applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks at the one of the plurality of specified intervals based on the at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the user operation of the apparatus has been initiated.
 9. A controller in communication with a non-volatile memory apparatus including a plurality of memory cells, each of the plurality of memory cells connected to one of a plurality of word lines and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to one of a plurality of data states, the plurality of memory cells operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up to a residual voltage level, the controller configured to: determine a power on event of the memory apparatus in which a voltage of 0 V or less is applied to the word lines, thereby causing the plurality of memory cells to be in the first read condition; instruct the memory apparatus to periodically apply a predetermined refresh read voltage, which is less than a programming voltage, to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the memory apparatus; and wherein the application of the refresh read voltage is based on at least one of a die temperature of a die of the memory apparatus and a failure bit count exceeding a predetermined failure bit count threshold.
 10. The controller as set forth in claim 9, wherein the at least one data retention factor includes the die temperature, the memory apparatus further includes a temperature sensor in communication with the controller and configured to measure the die temperature of the plurality of memory cells and a refresh frequency and temperature lookup table including the plurality of specified intervals each corresponding to one of a plurality of magnitudes of the die temperature, and the controller is further configured to: determine the die temperature of the plurality of memory cells using the temperature sensor; and determine and instruct the memory apparatus to utilize one of the plurality of specified intervals based on the refresh frequency and temperature lookup table.
 11. The controller as set forth in claim 9, wherein the memory apparatus further includes an error-correction code engine configured to determine and correct errors in reading the plurality of memory cells, the at least one data retention factor includes the failure bit count associated with reading at least a subset of the plurality of memory cells, and the controller is further configured to: instruct the memory apparatus to read the at least the subset of the plurality of memory cells; determine the failure bit count for the at least the subset of the plurality of memory cells using the error-correction code engine; determine whether the failure bit count exceeds a predetermined failure bit count threshold; and instruct the memory apparatus to apply the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the failure bit count exceeding the predetermined failure bit count threshold.
 12. The controller as set forth in claim 11, wherein the threshold voltage possible for each of the plurality of memory cells spans a threshold window, each of the plurality of memory cells is configured to store a plurality of bits, the plurality of data states includes an erased state at a first end of the threshold window and a plurality of programmed data states each corresponding to the threshold voltage being higher than the threshold voltage associated with the erased state, the plurality of programmed data states including one or more higher data states at a second end of the threshold window opposite the first end and associated with the threshold voltage being higher than the than the threshold voltage associated with the erased state and one or more other of the plurality of programmed data states, the at least one data retention factor includes a delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage between the one or more higher data states, and wherein the controller is further configured to: instruct the memory apparatus to read the at least the subset of the plurality of memory cells using a default read voltage associated with the one or more higher data states and using an adjusted read voltage being less than the default read voltage by a predetermined delta control gate voltage; determine the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage; determine whether the delta bit count quantity exceeds a predetermined delta bit count quantity threshold; and instruct the memory apparatus to apply the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the delta bit count quantity exceeding the predetermined delta bit count quantity threshold.
 13. A method of operating a non-volatile memory apparatus including a plurality of memory cells, each of the plurality of memory cells connected to one of a plurality of word lines and arranged in one of a plurality of blocks and configured to retain a threshold voltage corresponding to one of a plurality of data states, the plurality of memory cells operable in one of a first read condition in which a word line voltage of the plurality of word lines is discharged and a second read condition in which the word line voltage of the plurality of word lines is coupled up to a residual voltage level, the method comprising the steps of: determining a power on event of the memory apparatus in which a voltage of 0 V or less is applied to the word lines, thereby causing the plurality of memory cells to be in the first read condition; periodically applying a predetermined refresh read voltage, which is less than a programming voltage, to a selected one of the plurality of word lines for a predetermined period of time for each of the plurality of blocks at one of a plurality of specified intervals based on at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the power on event of the memory apparatus; and wherein the application of the refresh read voltage is based on at least one of a die temperature of a die of the memory apparatus and a failure bit count exceeding a predetermined failure bit count threshold.
 14. The method as set forth in claim 13, wherein the at least one data retention factor includes the die temperature, the memory apparatus further includes a temperature sensor configured to measure the die temperature of the plurality of memory cells and a refresh frequency and temperature lookup table including the plurality of specified intervals each corresponding to one of a plurality of magnitudes of the die temperature, and the method further includes the steps of: determining the die temperature of the plurality of memory cells using the temperature sensor; and determining and utilizing one of the plurality of specified intervals based on the refresh frequency and temperature lookup table.
 15. The method as set forth in claim 13, wherein the memory apparatus further includes an error-correction code engine configured to determine and correct errors in reading the plurality of memory cells, the at least one data retention factor includes the failure bit count associated with reading at least a subset of the plurality of memory cells, and the method further includes the steps of: reading the at least the subset of the plurality of memory cells; determining the failure bit count for the at least the subset of the plurality of memory cells using the error-correction code engine; determining whether the failure bit count exceeds a predetermined failure bit count threshold; and applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the failure bit count exceeding the predetermined failure bit count threshold.
 16. The method as set forth in claim 15, wherein the threshold voltage possible for each of the plurality of memory cells spans a threshold window, each of the plurality of memory cells is configured to store a plurality of bits, the plurality of data states includes an erased state at a first end of the threshold window and a plurality of programmed data states each corresponding to the threshold voltage being higher than the threshold voltage associated with the erased state, the plurality of programmed data states including one or more higher data states at a second end of the threshold window opposite the first end and associated with the threshold voltage being higher than the threshold voltage associated with the erased state and at least one other of the plurality of programmed data states, a data stored in the plurality of memory cells is stored in a plurality of pages respectively associated with each the plurality of bits stored by each of the plurality of memory cells and encoded according to an encoding scheme of the plurality of data states, the at least the subset of the plurality of memory cells includes at least one of the plurality of pages, and wherein the method further includes the steps of: selecting the at least one of the plurality of pages based on which of the at least one of the plurality of pages is associated with the one or more higher data states; reading the data stored in the at least one of the plurality of pages; and determining the failure bit count for the at least one of the plurality of pages using the error-correction code engine.
 17. The method as set forth in claim 16, wherein the plurality of bits includes four bits, the plurality of programmed data states includes, in order of the threshold voltage increasing in magnitude, a first data state and a second data state and a third data state and a fourth data state and a fifth data state and a sixth data state and a seventh data state and an eighth data state and a ninth data state and a tenth data state and an eleventh data state and a twelfth data state and a thirteenth data state and a fourteenth data state and a fifteenth data state, the plurality of pages includes a top page, an upper page, a middle page, and a lower page respectively associated with each the four bits stored by each of the plurality of memory cells and encoded according to the encoding scheme of the plurality of data states, and wherein the method further includes the steps of: applying a sequence of read voltages corresponding to the fifth data state, the tenth data state, the twelfth data state, and the fifteenth data state to the selected one of the plurality of word lines to read the data of the top page; and determining the failure bit count for the data stored in the top page using the error-correction code engine.
 18. The method as set forth in claim 13, wherein the threshold voltage possible for each of the plurality of memory cells spans a threshold window, each of the plurality of memory cells is configured to store a plurality of bits, the plurality of data states includes an erased state at a first end of the threshold window and a plurality of programmed data states each corresponding to the threshold voltage being higher than the threshold voltage associated with the erased state, the plurality of programmed data states including one or more higher data states at a second end of the threshold window opposite the first end and associated with the threshold voltage being higher than the than the threshold voltage associated with the erased state and at least one other of the plurality of programmed data states, the at least one data retention factor includes a delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage between the one or more higher data states, and wherein the method further includes the steps of: reading the at least the subset of the plurality of memory cells using a default read voltage associated with the one or more higher data states and using an adjusted read voltage being less than the default read voltage by a predetermined delta control gate voltage; determining the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage; determining whether the delta bit count quantity exceeds a predetermined delta bit count quantity threshold; and applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks in response to the delta bit count quantity exceeding the predetermined delta bit count quantity threshold.
 19. The method as set forth in claim 18, wherein the plurality of bits includes four bits, the plurality of programmed data states includes, in order of the threshold voltage increasing in magnitude, a first data state and a second data state and a third data state and a fourth data state and a fifth data state and a sixth data state and a seventh data state and an eighth data state and a ninth data state and a tenth data state and an eleventh data state and a twelfth data state and a thirteenth data state and a fourteenth data state and a fifteenth data state, and wherein the method further includes the step of reading the at least the subset of the plurality of memory cells using the default read voltage associated with the fifteenth data state and using the adjusted read voltage being less than the default read voltage by the predetermined delta control gate voltage to determine the delta bit count quantity of the at least the subset of the plurality of memory cells having the threshold voltage greater than the adjusted read voltage and less than the default read voltage.
 20. The method as set forth in claim 13, further including the steps of: determining whether a user operation of the memory apparatus has been initiated, and interrupting periodically applying the predetermined refresh read voltage to the selected one of the plurality of word lines for the predetermined period of time for each of the plurality of blocks at the one of the plurality of specified intervals based on the at least one data retention factor to maintain the plurality of memory cells of the plurality of blocks in the second read condition in response to determining the user operation of the memory apparatus has been initiated. 